There are several types of photoelectric smoke detectors. Most detectors use only forward scattering detectors with a light source in the near infrared. Some detectors use a dual angle sensing chamber, which measures both the forward and backward light scattered from particles in order to gain some insight into particle size.
Some detectors use more than one wavelength of light. Others use a combination of angles and wavelengths. Some detectors use a photoelectric sensing chamber combined with heat, gas, or light sensing, i.e., multi-criteria smoke detectors. One example of a photoelectric smoke sensor is disclosed in U.S. Pat. No. 6,521,907, entitled “Miniature Photoelectric Sensing Chamber,” which issued Feb. 18, 2003. One example of a multi-criteria detector is disclosed in U.S. Pat. No. 6,967,582, entitled “Detector With Ambient Photon Sensor and Other Sensors,” which issued Nov. 22, 2005. Both U.S. Pat. No. 6,521,907 and U.S. Pat. No. 6,967,582 are owned by the assignee hereof and incorporated herein by reference.
Photoelectric smoke sensors that use near infrared light (850 to 950 nm) are generally known to be better at detecting smoldering fires since those types of fires produce larger particles. Ionization-type smoke sensors tend to detect flaming fires better. Ionization sensing chambers are better at detecting small particles produced by the flaming fires. Ionization-based detectors are falling out of favor due to increased environmental regulations.
Smoke detectors are commercially available that use blue light emitting diodes (LED's). When blue LED's are used in forward scattering photoelectric smoke sensing chambers, a sensor's response to small particles improves. This is predicted by the Mie scattering theory, which says that particles will preferentially scatter light in the forward direction when the wavelength of light approaches the particle size. Small particles are typically produced by flaming fires.
At least some known photoelectric smoke sensors include an optic block that carries a light source, such as an LED, and a light sensitive element, such as a photodiode. The source and the light sensitive element are arranged at a prescribed angle to one another in order to detect scattered light. A housing surrounds the optic block and serves to exclude ambient light and direct the flow of ambient airborne particulate matter.
MOS (metal oxide semiconductor) gas sensors are typically heated to 200 to 400° C. for proper operation. This required heating can be achieved by using a resistance heater, causing high power consumption. Some thick film MOS gas sensors draw up to 500 mW, while thin film or MEMS devices may draw an order of magnitude less. This high power consumption limits the number of applications where they can be used. For example, system connected fire detectors require low power consumption due to battery backup requirements in the National Fire Alarm Code.
MOS gas sensors also tend to not be selective to one gas, but sensitive to a whole class of gases, e.g., oxidizing gases. Radiant energy can be directed onto such sensors to increase their sensitivity instead of heating them. Doing so reduces the amount of power required to operate them.